1. Field of the Invention
The present invention relates to a commutator for use in a rotating machine such as an electric motor and having a plurality of commutator segments or bars of a conductive material arranged around a sleeve molded of a thermosetting synthetic resin. It also relates to a method of making such a commutator.
2. Description of the Prior Art
Commutators are used in various rotating machines such as DC motors having brushes and they are composed of a plurality of commutator segments or bars arranged in a drumlike cylinder and supported by a commutator sleeve molded of a thermosetting synthetic resin, with air gaps or grooves defined between adjacent ones of the commutator bars and the outer periphery of the commutator sleeve. The sleeve has a finished central bore for accommodating the armature shaft.
In the manufacture of the commutators of the foregoing construction, it is customary practice to fill an internally and circumferentially grooved pipe of an electrically conductive material such as copper with a thermosetting synthetic resin, thereby forming a commutator blank. After the molded thermosetting synthetic resin is aged for dimensional stability, the central bore of the commutator blank is finished by reaming to correct a dimensional change which may have been caused by aging. Thereafter, the copper pipe is separated into a plurality of circumferentially spaced commutator segments or bars which are electrically separated from one another by undercuts or grooves defined between the adjacent ones of the commutator bars. Finally, the outer peripheral surface of the commutator blank is finished by cutting or turning so as to provide an improved commutation.
The commutator thus produced is assembled with the armature of a motor. When the motor is operating, the outer peripheral surface of the commutator is heated at a high temperature due mainly to the Joule heat resulting from the Joule effect produced by an electric current in the commutator bars and brushes bearing thereagainst, and the friction heat resulting from sliding contact between the commutator and the brushes. A part of the heat thus produced is transmitted from the commutator bars to the commutator sleeve and gradually deteriorates the thermosetting synthetic resin constituting the commutator sleeve. With this deterioration by heat, the commutator sleeve tends to create cracks in the reamed surface of the central bore and in the vicinity of the exposed surfaces between the adjacent commutator bars. The cracks thus created considerably lower the bonding strength between the commutator bars and the commutator sleeve and sometimes produce a step or difference in level between the adjacent commutator segments. Consequently, as the armature revolves, the brushes impinge against the stepped portion of the commutator and hence produce unpleasant vibration and noise and sparks during commutation. Under such condition, the brushes have only a short service life.
Various attempts have been made to overcome the foregoing problems. One such prior attempt is disclosed in Japanese Patent Publication No. 55-15948 in which the adjacent commutator bars are integrally connected by a special glass. Japanese Patent Laid-open Publication No. 59-209040 shows another prior attempt which involves the use of a commutator sleeve formed of a ceramic material instead of a resin material. According to a further attempt known from Japanese Patent Laid-open Publication No. 63-69446, a bushing adapted to be fitted over the armature shaft and commutator bars are integrally molded in concentric relation via an intermediate resin layer, with the bushing double-insulated from the resin layer.
The prior attempts disclosed in the first- and second-mentioned Japanese Publications are however still unsatisfactory in that the structure of the commutator and the process of making the commutator are rendered complicated because the commutator core is formed of a specific material other than the forming resin such as a thermosetting synthetic resin. The prior attempt according to the last-mentioned Japanese Publication has an advantage that the bushing is effective to prevent carbonization of the resin layer and leakage or insulation failure resulting from moisture absorption by the resin layer and various fillers. This attempt however still has a problem that the oxidative degradation of the molding resin and the cracking of the molding resin cannot be avoided.